In terms of power production, shadows are considered problematic, as their occurrence undermines the performance of solar photovoltaic arrays. Researchers from the National University of Singapore view shadows as a potential energy resource to be harnessed and have designed a simple device structure that makes use of shadows for power generation and photosensing applications.

The low cost, easy-to-fabricate shadow-effect energy generator (SEG) converts illumination contrast by partial Schematic illustration of the SEG; inset shows the device partly in shadow. Source: Royal Society of Chemistryshadow castings to electricity and can serve as a self-powered proximity sensor to monitor passing objects. As an energy harvester, arrays of SEGs adhered to a flexible substrate can be attached to clothing and can power wearable electronic devices by continuously converting the shadows cast on the SEG into electricity.

The SEG cell is composed of a thin gold film deposited on an n-type silicon wafer. The cells can be arranged in parallel to form arrays at fabrication costs that are lower than those for commercial silicon solar cells. Tests conducted with a four-cell array demonstrated significant electrical output when part of the SEG system is illuminated. Illuminating just half of the surface area of the device provides sufficient surface area for charge generation and collection, and the generator was documented to deliver a power density of 0.14 μW/cm2 under indoor conditions of 0.001 sun, where shadows are persistent.

When exposed to shifting shadows, the prototype proved twice as efficient as commercial solar cells, and the SEG supplied 1.2 V in the presence of shadows under indoor lighting conditions, or enough to power a digital watch. The SEG offers a promising technology for renewable energy generation from ambient conditions to power electronics and as a part of smart sensor systems, especially in buildings.